Electric altimeter



June lo, J. L HATHAWAY v ELECTRIC ALTIMETER Filed Feb. 28, 19:59

ffm' M576@ v Ma/Nee ttomeg Mwzfzawg Patented June l0, 1947 ELECTRIC ALTIMETER Jarrett L. Hathaway, Manhasset, N. lY., assignor to Radio Corporation of America, al corporation of Delaware Application February 28, 1939, Serial No. 259,066

1 14 claims. (ci. 17T-352i' This invention relates to aircraft altimeters in which a wave propagating method for measur` ing long distances is combined with a balancing system especially adapted for measuring short distances.

Various systems have'beenproposed for measuring the altitude of aircraft. I n one system, .waves are transmitted from the aircraft and the time between the'transmission of the wave and its reception after' reflection fromv the surface ofthe earth is indicated. Since the velocity 'of transmission isknown, the distance traveled may be readily determined. In one system of this type, the wave energy consists of discrete pulses. In another system, the wave energy is Y la radio frequencyrwave which is continuously changed in frequency so that the change of frequencybetween the transmitted and reected waves indicates altitude. Systems of both types are lespecially adaptable for the measurement of altitudes of the order of 50 feet and upwards. These systems become less and less suitable as the altitude diminishes because the time intervals become extremely short.

There are other systems in which the change of capacity, or change of impedance, is measured l as a function of the altitude above the surface which brings about the change. These systems,

'which depend upon a change in the balance condition, are especially suitable to determine low altitudes; for less.

It is v'an object of this invention to combine in a single indicating instrument the desirable characteristics of systems adaptable in the measurement of high altitudes with systems especially suitable in determining low altitudes. It is a further object of this invention to utilize apparatus which will have a common 'function in the measurement of the altitude of an aircraft or the like. A further object is to provide means for indicating the altitude of an aircraft with respect to the nearest object and to record the altitude of the aircraft as it travels along a course. A still further object Ais to operate a warning signal when the altitude of the aircraft reaches a predetermined limit.

The invention will .be described by reference to the accompanying drawing in which:

Figure 1 is a schematic representation of one embodiment of the invention; Figure 2 is a, graph indicating rthe Wave form of the currents which are applied to the indicator; and Figure 3 is a. graph illustrating the characteristics of the altimeter.

example, altitudes of 100 feet orY Referring to Fig. 1, a low frequency oscillator I is connected to a pulse generator and modulator 3.v The pulse generator is effectively connected to an ultrahigh frequency oscillator 5 whose output circuit is connected to an antenna array 1. The antenna preferably'includes a reector or director 9 for making the transmission as directive as practical. A second directive antenna I Il is connected to a radio receiver II which includes a limiterv which may consistof a thermionic tube operating at cut-oli? or saturation grid bias. The output of the radio receiver is applied to a combiner I3 which, in turn, is connected to a peak meter I5. The combiner may be a thermionic tube upon whose input circuit are applied serially the voltages to be combined.

The pulse generator and modulator 3 is con nected to a timing wave generator I1 which is connected to the combiner I3. The pulse generator is also connected tothe radio receiver II.

The low frequency oscillator is connectedto a Wheatstone A. C. capacity bridge I9 which is in turn connected to the combiner I 3.. Any warn' ing signal 2| and any recorder 23 may be connected in parallel with the peak meter.

The operation of the system is essentially as follows: 'I'he low frequency oscillator may genersfo ate sinusoidal currents which may have a frequency, for example, of 20kcs. -These currents .are applied to the pulse generator in which a sharply ydefined pulse is generated for each cycle of the low frequency oscillator. This pulse has a duration of the order of one-tenth microsecond. The pulse is applied to key the ultra high frequency oscillator so thata discrete pulse of radio frequency energy is directively radiated from the antenna l toward the earth or other-object 25. The pulse of high frequency energy is reflected and induces electromotive forces in the 'receiving antenna I0 which cause currents to flow to the receiver I I. The connection from the pulse generator to the receiver is to partially blank the receiver during the pulse transmission. This is not required if sufficient antenna directivity is maintained so that the received pulse may Ibe satisfactorily limited. The receiver includes means which limit the amplitude of the pulses. The timing wave generator operates in synchronism with the pulse generator and establishes timing potentials having a wave form 21 or 35 such as is indicated in Fig. 2. The'currents or potentials from the timing wave generator and the receiver are added in the combiner I3 and hence applied to the peak meter .the potentials corresponding I5. The peak meter indicates the sum of the potentials corresponding to the timing wave and the limited pulse 29.

In this system, the initial pulse will be indicated and later reflections of the initial pulse will be added to the potentials representingthe timing wave. For low altitudes the reflected pulse 29 willcombine with the low potential part of the curve 21 and will indicate the altitude as afunction of the peak voltage which is read on the peak meter l5. As the altitude increases, therefiected pulse 29 will return after a longer interval and hence will be combined nearer thepeak of the timing curve. As the altitude approaches low altitudes, it will be very diiilcult to distinguish any change in the peak meter reading as the reected pulses will appear close to the peaks of the initial'pulses. of 100 feet or more, the capacity bridge, which includesl capacitor plates 3 i, 33, is preferably balanced. As the altitude diminishes, become unbalanced and the resulting voltages 36, which arepreferably rectified and filtered, will be subtracted inthe combiner I3 to thereby in,

dicate diminishing altitudes on the vpeak meter.

The closer the capacity plates 3|, 33 are to the At an altitude of the order the bridge Will indicating altitudes as a function of the time r`edetermined,` means for earths surface, the more unbalance will be noted in the vbridge circuit, and the more voltage subtracted in the combiner. However, the balance condition may be arranged for zero altitude and to unbalance added in the combiner. y

The desirability of combining a capacity or balance type of altimeter with a timing type laltimeter can be graphically demonstrated. Refer ring to Fig. 3,'the curve C indicates the characteristic of a capacity altimeter. It will be observed that very little change of reading occurs. from very high altitudes down to 100 to 200 feet.'

From 200 feet downward the readings readily indicate small changes in altitude. On the other hand. the timing type altimeter curve T. while sufficiently accurate at the higher altitudes, is not adapted to indicate altitudes below about 1,00 feet with high accuracy. If the two curves are combined, the resultant curve A provides a characteristic which is extremely accurate for low altitudes and sufdciently reliable for the higher altitudes.

Thus the invention has been described as an altimeter in which high altitudes are Vdetermined by propagating discrete pulses of radio frequency energy and in which'low altitudes are measured by the balancing action of an electrical bridge.

No attempt has been made to describe the details of the various component parts, as all of these parts are well known to those. skilled in the art. It should be understood that the invention is not limited to the pulse method ofV determining the higher altitudes. as the system will work equally well if a vfrequency modulating system, suchA as described by Bentley, 2,011,392, is substituted for the pulse method. It should also be understood that the capacity bridge may be of the self-balancing type described in U. S. Patent 2,280,725, granted on April 2l, 1942, to F. H. Shepard, Jr., on his application Serial No. 243,171, filed November 30, 1938, and entitled Self-balancing capacity altimeter, or, in place of a capacity'bridge, an impedance measuring method of the type described by Alexanderson, U. S. Patent ,'No.

1,913,148 or Drake U. S. Patent No. 1,987,587 Vmay be used. The electrical timing wave generator may be designed to give a saw-tooth output, or a be used. The shape of the hyperbolic curve may flection from the earths surface, means for limit- A y ing the amplitude of'the received pulses, means lfor combining the electrical timing waveand the quency energy toward erating an 'electrical c quired to propagate a wave toward the earth and back to the point at-'which the altitude is to be and means for'combining the indications of said first and second mentioned means to provide indications which are dependent on both of said means.

3. An altitude indicating device including means for transmitting discrete pulses of radio frequency -energy toward the earth. means for producing an electrical timing wave of the same frequency and in synchronism with said pulses. means for receiving said discrete pulses after relimited pulses, and means for indicating the peak potential of said combined wave and limited pulse.

4; In an altitude measuring device including means for transmitting modulated radio irequency energy toward the earth, means for generating an electricalvtiming wave of the same the modulation of said energy. means for balancing said capacity bridge as a function of altitude, 'means' for combining said electrical timing wave and the output circuit of said bridge, and means for indicating said combined output.'V

5. In an altitude measuring device including means for transmitting modulated radio frethe earth, means for genfrequency and in4 synchronism with the modulation of said energy. an a. c. capacity bridge. means for balancing said capacity bridge as a function of altitude, Ameans for combining said electrical timing wave'and'the output circuit of said bridge, and means for indicating the peak potential of said combined timing wave and the Vpotential in said bridge output circuit.

6. In anzaltimeter indicating device. a low frequency oscillator, means for deriving discrete pulses from said low frequency oscillator. an ultra high frequency oscilla-tor, means for applying said discrete pulses tokey said high frequency oscillator, means for radiating pulses of high frequency energy, means for receiving said pulses after reection, -means for limiting the received pulse, means for generating an electrical timing wave of the same frequency and in synchronism with'said pulses. means for combining vthe re-v A ceived limited pulses `and the timingwave, and means for indicatingv the peak potential of said combined limited pulse and timing wave.

7. In a device of the character of claim 6, a. capacity bridge, means for applying said low frequency oscillations to two of the terminals of said bridge, and means forV combining the output of said capacity bridge' with said electrical timing indicating altitude as a. function of the balance of an electrical bridge.'

an a.1'c. capacity bridge,

timing wave of the same l wave and said limited pulses, and means for indicating said combined outputs.

8. The method of indicating the distance from a point to a surfacev which includes radiating modulated radio frequency energy from said point toward said surface, receiving said energy after reflection, limiting the potential of said received energy, generating an electrical timing wave of the same frequency as said modulation of said energy in synchronism with said radiation, combining said limited potential with said timing Wave, and indicating the peak of said combination.

9. The method of indicating the distance from a point to a surface which includes radiating pulses of radio frequency energy from said point toward said surface, receiving said pulses after reflection, limiting the potential of said received pulses, generating an electrical potential timing wave of the same frequency as said pulses in synchronism with said radiation, combining said' limited potential with said timing Wave potential, and indicating the peak of said combined potential.

10. The method of indicating the distance froma point to a surface which includes radiating modulated radio frequency energy from said point toward lsaid surface, receiving said energy after reflection, limiting the potential of said received energy, generating an electrical timing Wave of the same frequency as said modulation of said energy in synchronism with said radiation, combining said limited potential with said timing wave, generating oscillatory currents, balancing said oscillatory currents at a predetermined altitude, deriving potentials resulting from unbalancing at other than said predetermined altitude, and combining said potentials with said timing wave to indicate said other altitude.

11. An altitude indicating device including an indicator, an electrical bridge, means for balancing said bridge at a predetermined altitude, means for deriving currents varying as a function of said balance, means for applying said derived currents to said indicator, means for transmitting toward the earth from an altitude to be determined radio frequency energy, means for receiving said i energy after reflection from the earth, means for timing said transmission to the earth and return to measure altitudes in excess of said predetermined altitude, and means for applying currents corresponding to said last mentioned altitudes to said indicator.

12. An .altimeter including means forradiating radio frequency energy, means for receiving said energy after reflection from a surface the distance of which is to be determined, means connected to said receiving means for indicating the distance of said surface whereby distances in excess of a predetermined minimum may be indicated as a function ofthe propagation time of said radiated and reflected energy, an electrical bridge including an element which varies as a function of distance from said surface and unbalances said bridge, means for deriving currents corresponding to said bridge unbalance, and means for applying said derived currents to said indicating means to indicate distances less than said predetermined minimum.

13. An altimeter including in combination means for radiating radio frequency energy toward and receiving said energy after recction from a reflecting 'surface for determining distances greater than a predetermined minimum, an electrical bridge including an element varying as a function of distances less than said minimum, and a common indicator responsive respectively to said means and ,to said bridge for indicating distances greater than and less than and including said predetermined minimum.

14. Altitude measuring apparatus comprising, in combination, a radio frequency timing type altimeter, a reactive bridge type altimeter and a common indicator responsive simultaneously to both said timing type and said bridge type altimeters.

- JARRETT L. HATHAWAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED `STATES PATENTS 

